1. Field of the Invention
The present invention relates to a direct color thermal printing method, and more particularly to a method of preventing yellow color stains.
2. Description of the Related Art
A direct color thermal printing method directly develops colors on a color thermosensitive recording sheet by heating it with a thermal head. As disclosed, for example, in U.S. Pat. No. 5,268,707, a color thermosensitive recording sheet has a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer, respectively laid on a base in this order from the bottom. Each thermosensitive coloring layer has a different heat sensitivity in order to selectively develop colors on each thermosensitive coloring layer. The uppermost yellow thermosensitive coloring layer has a highest heat sensitivity, and the lowermost cyan thermosensitive coloring layer has a lowest heat sensitivity. In order not to develop colors on an overlying already colored thermosensitive coloring layer when the underlying thermosensitive coloring layer is colored, the already colored thermosensitive coloring layer is optically fixed by applying particular electromagnetic rays thereto.
Each heating element of a thermal head heats a color thermosensitive coloring sheet at a coloring heat energy (mJ/mm.sup.2) sufficient for obtaining a desired coloring density which heat energy is determined by a characteristic curve specific to each thermosensitive coloring layer. An ink dot is therefore recorded in each pixel having a virtually partitioned square area on a color thermosensitive recording sheet. This coloring heat energy is a sum of a heat energy having a level slightly short of starting coloring (hereinafter called a bias heat energy) and a heat energy for coloring at a desired density (hereinafter called an image heat energy). The bias heat energy has a constant level determined by the type of a thermosensitive coloring layer, whereas the image heat energy changes with image data representing a tonal level.
With a color thermal printer adopting a direct color thermal printing method, a thermal head and a color thermosensitive recording sheet are moved relatively to record a full-color image by sequentially printing three color images. For example, a platen type color thermal printer has a thermal head extending in a main scan direction and a platen drum rotating intermittently or continuously in a subsidiary scan direction. This platen drum is constituted by a metal shaft and a drum made of black hard rubber and fixed to the metal shaft. A color thermosensitive recording sheet is wound on the circumferential wall of the drum. As the platen drum is rotated and the color thermosensitive recording sheet is moved in the subsidiary scan direction, the thermal head presses and heats the print area of the color thermosensitive recording sheet. As soon as the back end of the print area passes under the thermal head, the thermal head is moved upward to detach it from the color thermosensitive recording sheet.
During the first rotation of the platen drum, the thermal head heats a color thermosensitive recording sheet to print a yellow image one line after another on the uppermost yellow thermosensitive coloring layer. After the yellow image is printed, ultraviolet rays having an emission peak of a wavelength of 420 nm are applied to the color thermosensitive recording sheet to optically fix the yellow image. Only a diazonium salt compound still not developing color in the yellow thermosensitive coloring layer is optically decomposed and the yellow thermosensitive coloring layer loses its coloring ability. During the second rotation of the platen drum, the thermal head heats a color thermosensitive recording sheet by a heat energy larger than printing the yellow image to sequentially print a magenta image one line after another on the magenta thermosensitive coloring layer. After the magenta image is printed, ultraviolet rays having an emission peak of 365 nm are applied to the color thermosensitive recording sheet to remove the coloring ability of the magenta thermosensitive coloring layer. During the third rotation, the thermal head heats the color thermosensitive recording sheet at the highest heat energy to print a cyan image one line after another on the cyan thermosensitive coloring layer.
If a cyan bias heat energy slightly short of starting coloring is applied to the white blank area of the color thermosensitive recording sheet not designated for recording of an image, the blank area changes to a light yellow colored area. This phenomenon is called yellow stains. Before the cyan printing process, the blank area has been optically fixed after the yellow and magenta printing. With this fixing processes, impurities are generated. These impurities are generally decomposed and removed in four to five hours. However, if a large heat energy is applied to impurities, they are thermally fixed and become light yellow substances which are yellow stains.
Although yellow stains in a half tone image are unobtrusive, those in an image having characters (such as title characters and compliments sentences) printed in black or other colors in a blank area or those in a binary image such as line drawings are obtrusive and the print quality is degraded. If a thermal head is longer than the lateral side of a print area, some heating elements face the outer area of the half tone print area. Although these heating elements are supplied with image data "0" and do not perform an image heating, they perform a bias heating like the other heating elements. Therefore, the bias heating of the cyan printing process generates yellow stains on the blank frame in the outside of the print area. The gloss of the blank frame is also reduced and degraded by a high temperature bias heating. In the case of a postcard having a half tone image area and a binary image area, yellow stains formed on the binary image area lower the finished quality.
A friction coefficient between a color thermosensitive recording sheet and a thermal head changes with the heat energy generated by a thermal head as shown in FIG. 13, assuming that the force of pressing the color thermosensitive recording sheet by the thermal head is constant. A friction coefficient becomes low as the temperature of the thermal head rises. With a small friction coefficient, the feed load of the color thermosensitive recording sheet becomes small. A thermal head is generally powered to print an image after it is pressed against a color thermosensitive recording sheet. Therefore, the feed loads before and after powering are different. As the feed load changes, the rotary shaft of a platen drum is twisted, the hard rubber of the platen drum is deformed, or the drive belt for rotating the platen drum is elongated or shortened. These recoverable status change is collectively called a sheet feed system distortion, for the purpose of description simplicity.
A distortion amount of the sheet feed system is determined by the sheet feed load. If the sheet feed load is constant, a color thermosensitive recording sheet can be fed at a desired speed and with the constant distortion amount corresponding to the feed load. However, if the sheet feed load changes, the sheet feed system distortion changes correspondingly. As the distortion amount changes, the feed speed of a color thermosensitive recording sheet changes temporarily. When and after the thermal head is powered, the distortion amount of the sheet feed system reduces temporarily. As a result, the feed speed of a color thermosensitive recording sheet increases temporarily, the width of a printed line is broadened, and the coloring density lowers.
The coloring heat energy of a color thermosensitive recording sheet differs between colors so that the friction coefficient also differs between colors. Since the distortion amount of the sheet feed system becomes different between colors, a color registration shift occurs lowering the print quality.